Amplifiers are one of the main building blocks of any system that receives or transmits signals. They may be used to amplify a received signal to increase the signal strength and improve the signal to noise ratio (SNR), and may be used in transmitter chains to amplify signals to acceptable levels to drive antennae. They may also be used between system blocks to amplify a signal as desired.
A typical amplifier includes one or more transistors. These transistors could be any kind (bipolar, MOSFET, MESFET, JFET, etc.) and can be of any mode (depletion or enhancement type), depending on the specific needs of the application. Regardless of the type and mode of the amplifier, the transistor used in the amplifier should be biased at its gate or its base in order to achieve active operation. The bias point affects many parameters of the amplifier such as, efficiency, noise figure, linearity, stability and gain. However, every transistor is different from each other. Their threshold (pinch-off) value varies from part to part due to process variation, as well as aging and temperature. In order to compensate for these changes, quiescent bias through amplifiers should be monitored and their gate or base voltage preferably adjusted constantly to achieve stable operation.
An active bias control is a feedback system that typically measures the quiescent bias through a transistor and adjusts the gate or base of the amplifier to achieve a stable bias point. Many system level and on-chip active bias control loops have been designed and published over the years.
U.S. Pat. No. 5,361,007 to Ohta discloses an apparatus for generating a stable amplifier gate voltage by sensing the current passing through the amplifier by using a sense resistor in series with the amplifier (connected to the drain or collector) and using feedback to keep the voltage across this sense resistor constant. U.S. Pat. No. 5,311,143 to Soliday discloses another sensing apparatus which connects the sense resistor to the emitter of the amplifier rather than the collector. U.S. Pat. No. 6,657,499 to Lin discloses a bias generation circuit that uses a sense resistor connected in series with the amplifier.
These prior systems described above use a sense resistor in series with the RF amplifier which is desirable to avoid since the total quiescent current of the amplifier is also passing through the sense resistor thereby dissipating extra power. Accurate current setting requires very accurate trimming of the sense resistor. If the quiescent current of the amplifier is large (>1 A, which is the case for driver amplifiers) the sense resistor should be large enough to handle the current as well as accurate enough not to cause part to part variation.
Another important consideration for amplifiers is the proper power-up/power-down sequence to be applied to the amplifier. If a drain voltage is applied before the bias generator is properly turned on, excessive current will pass through the amplifier, possibly damaging it.